Dual-purposing modifier keys in xkb?

[Seth House]

Is it possible to dual-purpose modifier keys with xkb (or anything else)?

For example, if I press ctrl-f it emits ctrl-f but if I simply press and release
ctrl it sends some other keycode, such as esc.

I spent several hours combing through xkb documention yesterday but it is dense
and slow-going. The clearLocks flag docs seem to hint that xkb can tell the
difference between a straight press-and-release versus a
hold-<some-other-keypress>-release. Can anyone guide me in the right direction?

Thanks.
- Seth